Bone marrow fat is not all that for assessing skeletal changes

Dec 7, 2004

Bone marrow fat is a characteristic of bone quality that, independent of bone mineral density (BMD), contributes to skeletal strength. Research has suggested that increased bone marrow fat may present a fracture risk. But is there a direct connection between bone marrow fat and the potential loss or gain of BMD? A group from Georgetown University Hospital in Washington, DC, set out to answer that question using ¹H MR spectroscopy and bone densitometry (DEXA).

"Until recently, researchers had to assess bone marrow fat with invasive methods.... Proton MR spectroscopy (¹H MR spectroscopy) can gauge bone marrow fat noninvasively and serially," Dr. Dieter Schellinger and colleagues wrote in a paper published in the American Journal of Roentgenology (December 2004, Vol. 183:6, pp. 1761-1765).

In particular, the group looked to see if BMD and bone marrow fat are directly related, making the former a substitute measure for the latter. Also, if BMD and bone marrow fat are not interrelated, can bone marrow fat be an independent marker of bone quality?

Their patient population consisted of those with normal and structurally weakened vertebrae. Twenty-three women and three men underwent DEXA tests on a 4000W scanner (Hologic, Bedford, MA) and MR scans on a 1.5-tesla system (Vision, Siemens Medical Solutions, Malvern, PA), usually at L1-L4 levels of the spine. On MR spectroscopy, the bone marrow had two dominant peaks of water and lipid.

The findings showed that bone marrow fat can indicate bone weakening nearly as well as BMD, although it cannot be used as an independent predictor of weakening. In addition, the authors did not find a meaningful, inverse relationship between bone marrow fat and BMD, making bone marrow fat an unfit proxy for BMD.

However, the bone marrow fat to BMD ratio did show significant diagnostic power for detecting bone weakening. On MRI, 15 subjects had normal-appearing vertebrae, while 11 showed MRI stigmata of bone weakening, the group said.

"Although intact vertebrae in the 'healthy' subject group still may harbor osteoporosis, imaging stigmata of bone weakening probably signify a greater degree of bone loss," they wrote. This relationship may be because excessive amounts of bone marrow fat could reflect diminished bone mass. Also, adipogenesis competes with osteogenesis by reducing osteoblasts, they hypothesized.

The group stressed that this was a pilot study with a small population, and that the findings must be confirmed with a larger patient population. A 2000 consensus statement on osteoporosis by the National Institutes of Health (NIH) suggests that diagnosis and treatment may shift more toward risk-based assessment rather than solely on T-score supplied by the DEXA scan.

"Consideration of risk factors in conjunction with BMD will likely improve the ability to predict fracture risk," according to the statement (Osteoporosis Prevention, Diagnosis, and Therapy, NIH consensus statement online, March 27-29, 2000).

By Shalmali Pal
AuntMinnie.com staff writer
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